Methods and apparatus for calibrating digital imaging devices

ABSTRACT

Methods and apparatus are provided for calibrating a digital color imaging device to a printing press by determining a total colorant limit, per-channel colorant limits, and channel linearization tables using calorimetric and/or spectrophotometric techniques. In addition, for digital color imaging devices that use multi-hue colorants, methods and apparatus are provided for determining distribution functions for the multi-hue colorants as a function of input values.

BACKGROUND

This invention relates to color image processing. In particular, thisinvention relates to methods and apparatus for calibrating digital colorimaging devices using colorimetric or spectrophotometric techniques.Digital color imaging devices, such as digital color printers and colorcopiers, have become increasingly popular in recent years. Indeed, whilethe cost of digital color imaging devices has dropped significantly, thenumber of hardware devices and software applications that are capable ofproducing color output that may be printed on such devices hassubstantially increased. Further, as the output quality and resolutionof digital color imaging devices has improved, the number of uses forsuch devices has further increased.

For example, digital color laser printers and color inkjet printers noware increasingly used as relatively low cost proofing devices forcommercial printing presses. Printing a print job on a printing press isa fairly expensive and time-consuming process. As a result, mistakes orerrors in the print job are expensive to correct once a press run hascommenced. To minimize such costly errors, high quality inkjet printersmay be used to provide a proof of the print job before going to press.Ideally, the output of the proofing printer will visually match theoutput of the press. As a result, the proof output may be used forpurposes of approving the print job or making any necessarymodifications to the print job before printing the job on the press.

Referring now to FIG. 1, a previously known printing and proofing systemis described. Printing system 20 includes commercial printing system 22and proofing system 24. Commercial printing system 22 includes inputdevice 26, input profile 28, color processing stage 30, press profile 32and press 34. Input device 26 may be any device that may be used tocreate and/or store color image 38. For example, input device 26 may bea color scanner, digital camera, computer workstation, computer memoryor other similar device.

Color image 38 includes a bitmap array of pixels, with each pixelincluding multiple colorant values. For example, if input device 26 is ascanner, color image 38 may include pixels expressed as a combination ofred, green and blue (“RGB”) colorants. Colorant values typically arerepresented as multi-bit digital data values. Thus, if eight bits areused for each colorant, the colorant values may range from 0-255. Inthis regard, 0 corresponds to no colorant, and 255 corresponds to 100%colorant. The colorant values of color image 38 are defined in thedevice-dependent color space of input device 26.

Input profile 28 includes transformations between the color space ofinput device 26 and a profile connection space, such as CommissionInternationale de l'Eclairage (“CIE”) XYZ, or other similar profileconnection space. A profile connection space derived from the XYZ colorspace is commonly known as the CIE LAB color space, which expressescolor values in a rectangular coordinate system, with the L, a, and bvalues each corresponding to one of the three dimensions in the system.The L-value characterizes the lightness aspect of the region along anaxis ranging from black to white, with corresponding values ranging from0 to 100. The a-value characterizes the color of the region along anaxis ranging from green to red, with positive values corresponding tored and negative values corresponding to green. The b-valuecharacterizes the color of the region along an axis ranging from blue toyellow, with positive values corresponding to yellow and negative valuescorresponding to blue. Together, the a-value and the b-value may be usedto express the hue (“H”) and chroma (“C”) of the region:

$H = {\tan^{- 1}\left( \frac{b}{a} \right)}$ $C = \sqrt{a^{2} + b^{2}}$The zero point in the plane defined by the a-values and the b-valuescorresponds to a neutral gray color having an L-value corresponding tothe intersection of the plane with the L-axis.

Input profile 28 typically is produced in accordance with the profilespecification of the International Color Consortium (“ICC”), and henceis referred to as an “ICC profile.” An ICC profile generally includes atransform from the profile connection space to the device space (the“forward transform”), and a transform from the device space to theprofile connection space (the “backwards transform”). An input profile,however, typically includes only a backwards transform. For example, ifinput device is an RGB scanner, the backwards transform of input profile28 may be used to convert device-dependent RGB colorant values toequivalent device-independent LAB colorant values.

Color processing stage 30 optionally may be used to perform variouscolor processing operations in device-independent color space. Forexample, color processing stage may include software used to performcolor editing or other color processing operations. Press profile 32includes transformations between the color space of press 34 and aprofile connection space, and also is typically an ICC profile. Thus,press profile 32 typically includes forward transform 32 a and backwardstransform 32 b. For example, if press 34 is a conventional four-coloroffset press that uses cyan, yellow, magenta and black (“CMYK”)colorants, forward transform 32 a may be used to convertdevice-independent LAB colorant values to equivalent device-dependentcolorant values CMYK₁. Press 34 receives CMYK₁ colorant values andprovides press output 36 on media designed for use by a printing press.

Proofing system 24 includes press profile 32, printer profile 40,calibration stage 42 and proofing printer 46. In particular, backwardstransform 32 b of press profile 32 may be used to convertdevice-dependent colorant values CMYK₁ to device-independent LABcolorant values. Printer profile 40 is typically an ICC profile, andincludes a forward transform between the profile connection space andthe color space of proofing printer 46. Accordingly, the forwardtransform of printer profile 40 is used to convert device-independentLAB colorant values to device-dependent colorant values CMYK₂.

Calibration stage 42 typically includes hardware and/or software that:(a) maps calibrated input values to equivalent uncalibrated input values(sometimes referred to as “linearization”); (b) limits the colorant ofeach channel; and (c) limits the total colorant of all channels. Ifproofing printer 46 uses multi-shade colorants, calibration stage 42also may convert single colorant input values to equivalent multi-shadecolorant values. The mapping and per-channel colorant limit functionstypically are performed using tables that are designed to match theoutput response of proofing printer 46 to the output response of press34, and also limit the colorant of each channel. The total colorantlimit function is used to limit the total amount of colorant that may beoutput by proofing printer 46 to avoid negative image artifacts causedby using excessive colorant. Proofing printer 46 may be a digital inkjetprinter, such as a CMYK inkjet printer or other similar printer.Proofing printer 46 receives calibrated CMYK colorant values andprovides printed output 48 on media designed for use by an inkjet orlaser printer.

The process of “calibrating” a printer typically includes determininglinearization table values, per-channel colorant limits, a totalcolorant limit (“TCL”) and, optionally, distribution functions formulti-shade colorants. Referring now to FIG. 2, a previously knownprinter calibration process 50 is described. Beginning at step 52, a TCLis determined. In a multi-colorant printer, the amount of colorant foreach channel typically is specified as a percentage between 0 and 100%.Thus, on a four-color printer, the maximum sum of all colorants that maybe specified is 400%, corresponding to 100% on all four channels. Ifexcessive colorant is used, however, undesirable image artifacts mayresult that produce an unacceptable print. For example, on inkjetprinters, excessive colorant may cause bleeding (an undesirable mixingof colorants along a boundary between printed areas of differentcolorants), cockling (warping or deformation of the receiving materialthat may occur from using excessive colorant), flaking and smearing. Insevere cases, excessive ink may cause the print media to warp so muchthat it interferes with the mechanical operation of the printer and maydamage the printer. Thus, at step 52, a TCL is determined to minimizethe effects of excessive colorant.

Previously known techniques for determining a TCL typically rely ontrial and error methods that may be unsuitable for proofing purposes. Inparticular, previously known techniques typically involve printingseveral color patches that include various combinations of total amountsof colorant. A user then visually inspects the resulting printed output,and selects the patch (and thus the TCL) that produces the “best”results. A problem with such previously known techniques, however, isthat the results may vary substantially from user to user, and even fromtime to time by the same user. The resulting lack of repeatabilityimpairs the goal of obtaining a highly accurate proof.

Referring again to FIG. 2, after determining a TCL, at step 54 acolorant limit is determined for each channel. In a conventionalprinter, such as a CMYK inkjet or laser printer, the chroma response ofthe C, M and Y colorants as a function of the colorant amount isquasi-linear. However, beyond a certain specified colorant amount, thechroma actually begins to decrease, and the chroma response becomeshighly non-linear. For the K channel, the luminance decreases withincreasing colorant amount, until the luminance reaches a minimum level,but further increases in the colorant amount produce no further decreasein luminance. Indeed, for some combinations of colorants and media,oversaturation may occur, in which printed colors do not become anydarker, and may actually become lighter, with increasing colorantamounts. Because it is difficult to accurately profile a printer in thenon-linear region of operation, previously known techniques forcalibrating a printer typically limit the colorant of each channel sothat the printer operates only in the quasi-linear region and not in theoversaturation region.

Previously known techniques for determining per-channel colorant limits,however, have typically relied on density-based measurements that may beincomplete and inaccurate for proofing purposes. In particular,previously known techniques for determining per-channel colorant limitstypically involve printing a target for each colorant, where the targetincludes several color patches that range from 0 to 100% colorant. Afterprinting the target, a user typically measures the optical density ofeach patch using a densitometer or other instrument that providesoptical density values. The per-channel colorant limits are thenspecified as the colorant values that produce a predetermined density(e.g., the lowest maximum density) on all channels.

One problem with such isometric density techniques is that they fail toconsider the impact of the colorant limitation on the gray balance ofthe printer. When a printer outputs approximately equal percentages ofC, M and Y colorants, a neutral gray should result. The human eye isvery sensitive to detecting shifts in neutrality when neutral areas arecompared side-by-side. Thus, gray balance may be used to determine ifthe gamut of one printing device (e.g., a proofing printer) matches thegamut of another printing device (e.g., a press). Previously knowndensity-based techniques for determining per-channel colorant limits,however, typically do not ensure proper gray balance. To solve thisproblem, experienced users have developed their own techniques forachieving a desired density value for each colorant and also a good graybalance. Such empirical techniques vary from user to user, however, andrequire specialized knowledge that all users may not possess.

In addition, previously known density-based techniques for determiningper-channel colorant limits may be inaccurate for proofing printers.Conventional densitometers typically operate by illuminating a printedpatch using light having a known spectral distribution, and thenmeasuring the amount of light absorbed in a narrow frequency band of thevisual spectrum. The absorption measurement may then be translated to adensity measurement, with higher absorption corresponding to higherdensity. Densitometers typically use narrow-band optical filters thatare tailored to describe the behavior of colorants used on aconventional printing press. Unfortunately, however, the filters are notoptimized for describing the behavior of colorants used by conventionalinkjet and laser printers used for proofing. Indeed, if a colorant usedby a proofing printer has a maximum absorption at a frequency outsidethe band of the instrument's filters, the resulting density measurementsmay be incorrect. As a result, density-based techniques for determiningper-channel colorant limits may produce inaccurate results.

Referring again to FIG. 2, after per-channel colorant limits have beendetermined, in step 56, linearization tables are calculated for eachchannel so that the output response of proofing printer 46 matches theoutput response of press 34. Previously known techniques for calculatinglinearization tables typically involve printing a target for eachcolorant that includes several color patches that range from 0 to 100%colorant coverage. After printing the target, a user typically measuresthe optical density of each patch using a densitometer or otherinstrument that provides optical density values, and then calculatestable values that map the input/output density response of the printerto an input/output density response of the press. As described above,however, conventional densitometers and similar measuring instrumentsmay not accurately measure density of colorants used by conventionalinkjet and laser printers used for proofing. As a result,previously-known density-based techniques for calculating linearizationtables may produce similarly inaccurate results.

Referring again to FIG. 2, after linearization tables have beencalculated, at optional step 58, distribution functions may bedetermined for multi-hue colorants. In particular, high-quality digitalinkjet printers used for proofing purposes often include four primaryCMYK colorants (also referred to herein as “normal cyan,” “normalmagenta,” “normal yellow” and “normal black”), plus light cyan and lightmagenta colorants (indicated by lowercase “c” and “m”) to provideimproved image quality in the highlight regions of an image. Referringagain to FIG. 1, if proofing printer 46 is a CcMmYK printer, printerprofile 40 typically converts LAB values to CMYK values, and calibrationstage 42 converts cyan values into mixtures of normal cyan (C) and lightcyan (c) values, and converts the magenta values into mixtures of normalmagenta (M) and light magenta (m) values.

Previously known techniques for converting a specified colorant value toequivalent multi-shade colorants often rely on trial and errortechniques to determine the distribution function between the colorants.The resulting distribution function may be acceptable for a first set ofcolorants (e.g., used in a first printer in location A), but may beunacceptable for a second set of colorants (e.g., use in a secondprinter in location B). As a result, unless a new distribution functionis determined for the second set of colorants, the printed output of thetwo printers may not match. Previously known trial and error techniques,however, typically do not permit easy modification of distributionfunctions. Instead, the entire process must be repeated, which may beextremely time consuming and inefficient.

In view of the foregoing, it would be desirable to provide apparatus andmethods for calibrating a digital imaging device in a repeatable manner.

It also would be desirable to provide apparatus and methods forcalibrating a digital imaging device in an accurate manner.

It additionally would be desirable to provide apparatus and methods forcalibrating a printer without requiring specialized knowledge by a user.

SUMMARY

In view of the foregoing, it is an object of this invention to provideapparatus and methods for calibrating a digital imaging device in arepeatable manner.

It also is an object of this invention to provide apparatus and methodsfor calibrating a digital imaging device in an accurate manner.

It additionally is an object of this invention to provide apparatus andmethods for calibrating a printer without requiring specializedknowledge by a user.

These and other objects of this invention are accomplished by providingmethods and apparatus for calibrating a digital color imaging device toa printing press by determining a total colorant limit, per-channelcolorant limits, and channel linearization tables using colorimetricand/or spectrophotometric techniques. In addition, for digital colorimaging devices that use multi-hue colorants, methods and apparatus ofthis invention optionally may determine distribution functions for themulti-hue colorants as a function of input values.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned objects and features of the present invention can bemore clearly understood from the following detailed descriptionconsidered in conjunction with the following drawings, in which the samereference numerals denote the same elements throughout, and in which:

FIG. 1 is a block diagram of a previously known printing system;

FIG. 2 is a flowchart of a previously known printer calibration process;

FIG. 3 is a block diagram of an exemplary calibration system inaccordance with this invention;

FIG. 4 is a flowchart of an exemplary process for determining a totalcolorant limit in accordance with this invention;

FIG. 5 is an exemplary test chart for use with the method of FIG. 4;

FIGS. 6A-6C are tables of exemplary colorant values and associatedcalorimetric measurements for the test chart of FIG. 6;

FIG. 7 is a flowchart of an exemplary process for determiningper-channel colorant limits in accordance with this invention;

FIGS. 8A-8C are flow diagrams of exemplary methods for determiningminimum, maximum and optimal per-channel colorant limits in accordancewith this invention;

FIG. 9 is an exemplary test chart for use with the methods of FIGS. 8;

FIGS. 10A-10G are tables of exemplary colorant values and associatedcalorimetric measurements for the test chart of FIG. 9;

FIGS. 10H-10I are tables of exemplary interpolated colorant values andcalculated chroma values in accordance with this invention;

FIG. 11 is a flowchart of an exemplary process for determining alinearization table in accordance with this invention;

FIG. 12 is an exemplary test chart for use with the method of FIG. 11;

FIG. 13 is a tables of exemplary colorant values and associatedcalorimetric measurements for the test chart of FIG. 12;

FIG. 14 is a diagram of exemplary tonal responses calculated inaccordance with the method of FIG. 11; and

FIG. 15 is a flow diagram of an exemplary process for determiningdual-tone distributions in accordance with this invention.

DETAILED DESCRIPTION

Referring now to FIG. 3, an exemplary system in accordance with thisinvention is described for calibrating a digital imaging device usingcolorimetric and/or spectrophotometric techniques. Calibration system 70includes image source 72, proofing printer 46, output pages 74,measurement device 76, press profile 32 and processor 78. Image source72 includes image file 80 that comprises digital data representing testpatterns 82 to be printed by proofing printer 46. Image source 72 may bea personal computer, laptop computer, handheld computer, computerworkstation, print server, personal digital assistant, or any othersimilar device that may be used to provide image files for printing bycolor imaging devices.

Image source 72 may include a software application (not shown) used togenerate image file 80. For example, image source 72 may be a personalcomputer that includes software that may be used to generate image file80. Image file 80 may be a digital data file that describes testpatterns 82 in a page description language, such as PostScript, PCL, orother similar page description language, or may simply be a rasterimage, such as a TIFF image, RAW image, or other similar raster image.Proofing printer 46 may be a laser printer, inkjet printer or othersimilar color imaging device that uses one or more colorants to provideoutput pages 74 including test patterns 82. For example, proofingprinter 46 may be a four-color inkjet printer that uses CMYK colorants,a six-color inkjet printer that uses CcMmYK, or other similarmulti-colorant imaging device. Test patterns 82 include one or morecolor patches P.

Measurement device 76 may be any conventional measurement device thatmay be used to provide spectral and/or colorimetric data that describesa printed sample, such as a colorimeter, spectrophotometer,spectrocolorimeter, or other similar device. For example, measurementdevice 76 may be a Spectrolino spectrophotometer manufactured byGretagMacbeth LLC, New Windsor, N.Y., U.S.A. Measurement device 76provides colorimetric data, such as CIE LAB data (referred to herein as“LAB data”), CIE XYZ data (referred to herein as “XYZ data”), CIE LCHdata (referred to herein as “LCH data,” where L-values correspond tolightness, C-values correspond to chroma, and H-values correspond tohue), or other similar colorimetric and/or spectral data that describesprinted samples, such as color patches P.

Processor 78 may be a personal computer, laptop computer, handheldcomputer, computer workstation, print server, personal digitalassistant, or any other similar device that may be used to receivecalorimetric data, such as LAB data (i.e., L-, a- and b-values), LCHdata (i.e., L-, C- and H-values), or other similar calorimetric and/orspectral data from measurement device 76 and generate therefromcalibration data. Persons of ordinary skill in the art will understandthat the functions of processor 78 may be implemented by image source72. In accordance with this invention, processor 78 determines totalcolorant limit, per-channel colorant limits, and channel linearizationtables (one table per colorant) using calorimetric and/orspectrophotometric techniques. For proofing printers that use multi-huecolorants (e.g., light and normal cyan), processor 78 may also determinedistribution functions for the multi-hue colorants as a function ofinput values. Each of these techniques will be discussed in turn.

Total Colorant Limit

Referring now to FIGS. 3 and 4, exemplary methods and apparatus inaccordance with this invention are described for determining a TCL ofproofing printer 46. Beginning at step 90, proofing printer 46 is usedto print test pattern 82 including test patches P on output page 74. Forexample, a user of image source 72 may issue a print command to printimage file 80 including test pattern 82 on proofing printer 46. Anexemplary output page 74 a including exemplary test pattern 82 a isillustrated in FIG. 5. Test pattern 82 a includes an array of threestrips A, B and C of test patches P, with each strip including sixteentest patches. Persons of ordinary skill in the art will understand thattest pattern 82 a may include more or less than three strips, and eachstrip may include more or less than sixteen test patches P. Each testpatch P is comprised of a corresponding specified percentage ofcolorants used by proofing printer 46 (e.g., C, M, Y and K).

FIGS. 6A-6C illustrate tables of exemplary colorant values (in percent)for test patches P in strips A, B and C, respectively (patches areidentified in each table by row (A, B or C) and column number (1-16)).The exemplary values provide 48 test patches P having total colorantvalues (i.e., the sum of the colorant percentages for each patch) thatrange from 99% to 300% for strip A, 100% to 200% for strip B, and 100%to 400% for strip C. In this regard, the 48 exemplary test patches Pcover a broad range of total colorant values between 99% and 400%.Persons of ordinary skill in the art will understand that other colorantvalues also may be used for test patches P, and that the range ofcolorant values may include values less than 100%. In addition, personsof ordinary skill in the art will understand that the arrangement ofpatches within strips A, B and C, and the arrangement of the stripswithin test pattern 82 a may be changed.

Referring again to FIGS. 3 and 4, at step 92, calorimetric values aredetermined for each test patch P printed in step 90. For example,measurement device 76 may be used to determine LAB data for each testpatch P on output page 74 a. FIGS. 6A-6C illustrate exemplary measuredLAB data for test patches P in test pattern 82 a. Referring again toFIG. 4, at step 94, a test patch P is identified in each strip A, B andC that has the minimum L-value of all of the patches in the strip. Thus,from the exemplary colorimetric values shown in FIGS. 6A-6C, the minimumL-value for strip A (10.53) corresponds to test patch A11, the minimumL-value for strip B (26.69) corresponds to test patch B9, and theminimum L-value for strip C (9.18) corresponds to test patch C9.

Referring again to FIG. 4, at step 96, the total area coverage (“TAC”)is determined for each of the patches identified at step 94. The TAC ofa patch equals the sum of the colorant values for the patch. Thus,referring to FIGS. 6A-6C, the TAC for exemplary test patches A11, B9,and C9 is 300%, 200% and 260%, respectively. Referring again to FIG. 4,at step 98, the TCL is set to the maximum TAC determined in step 96.Thus, for the exemplary colorant values shown in FIGS. 6A-6C, the TCL is300%.

Per-Channel Colorant Limit

In addition to determining TCL, methods and apparatus in accordance withthis invention also determine a limit for each colorant of proofingprinter 46, while seeking to maintain the gamut of the proofing printeras large as the gamut of press 34. Referring now to FIGS. 3 and 7,exemplary methods and apparatus in accordance with this invention aredescribed for determining per-channel colorant limits. Beginning at step100, a minimum limit is determined for each colorant. Next, at step 102,a maximum limit is determined for each colorant. Finally, at step 104,an optimal limit between the minimum and maximum limit is determined foreach colorant. Each of these steps will be described in turn.

Referring now to FIGS. 3 and 8A, an exemplary method 100 for determininga minimum per-channel colorant limit is described. In particular, atstep 110, proofing printer 46 is used to print test pattern 82 includingtest patches P on output page 74. An exemplary output page 74 bincluding exemplary test pattern 82 b is illustrated in FIG. 9. Testpattern 82 b includes an array of eight strips A-H of test patches P,with each strip including twenty-two test patches. Persons of ordinaryskill in the art will understand that test pattern 82 b may include moreor less than eight strips, and each strip may include more or less thantwenty-two test patches P. Each test patch P is comprised of acorresponding specified percentage of colorants used by proofing printer46 (e.g., C, M, Y and K).

FIGS. 10A-10G illustrate exemplary colorant values (in percent) for testpatches P (patches are identified in each table by row (A-H) and columnnumber (1-22)). The exemplary values provide test patches P havingscales of single-colorant values for each colorant, and patches havingvarious combinations of multiple-colorant values. In particular, FIG.10A illustrates exemplary colorant values for test patches A1-A17including a scale of cyan colorant from 100% to 28%; FIG. 10Billustrates exemplary colorant values for test patches A18-B10 includinga scale of magenta colorant from 100% to 36%; FIG. 10C illustratesexemplary colorant values for test patches B11-C2 including a scale ofyellow colorant from 100% to 40%; FIG. 10D illustrates exemplarycolorant values for test patches H1-H22 including a scale of blackcolorant from 36% to 100%; and FIGS. 10E-10G illustrate exemplarycolorant values for test patches C3-G22 including various combinationsof C, M and Y colorants. Persons of ordinary skill in the art willunderstand that other specific colorant values also may be used for testpatches P.

Referring again to FIG. 8A, at step 112, calorimetric values aredetermined for each test patch P printed in step 110. For example,measurement device 76 may be used to determine XYZ and LAB data for eachtest patch P on output page 74 b. FIGS. 10A-10G illustrate exemplarymeasured XYZ and LAB data (and corresponding chroma data) for testpatches P. Referring again to FIG. 8A, at step 114, calorimetric valuesassociated with maximum amounts of each colorant are extracted frompress profile 32. In particular, for cyan, magenta, yellow and black,the calorimetric values that correspond to maximum amounts of cyan(i.e., CMYK value of (100, 0, 0, 0)), magenta (i.e., CMYK value of (0,100, 0, 0)), yellow (i.e., CMYK value of (0, 0, 100, 0)) and black(i.e., CMYK value of (0, 0, 0, 100)), respectively, are extracted frombackwards transform 32 a of press profile 32. Table 1 illustratesexemplary calorimetric values extracted from exemplary press profile 32:

TABLE 1 CMYK L a b (100, 0, 0, 0) 54 −36 −50 (0, 100, 0, 0) 47 75 −7 (0,0, 100, 0) 88 −6 95 (0, 0, 0, 100) 18 1 −1

Referring again to FIG. 8A, at steps 116 a-d, the minimum limitsC_(MIN), M_(MIN), Y_(MIN) and K_(MIN) are determined for the C, M, Y andK colorants, respectively. In particular, at step 116 a, the minimumcyan limit C_(MIN) is identified as the colorant value of the minimumcyan patch that has a- and b-values whose magnitudes are greater than orequal to the magnitudes of the a- and b-values, respectively, extractedfor cyan from press profile 32. At step 116 b, the minimum magenta limitM_(MIN) is identified as the colorant value of the minimum magenta patchthat has: (a) a- and b-values whose magnitudes are greater than or equalto the magnitudes of the a- and b-values, respectively, extracted formagenta from press profile 32, or (b) an L-value that is less than theL-value extracted for magenta from press profile 32, whichever is lower.At step 116 c, the minimum yellow limit Y_(MIN) is identified as thecolorant value of the minimum yellow patch that has a b-value whosemagnitude is greater than or equal to the magnitude of the b-valueextracted for yellow from press profile 32. At step 116 d, the minimumblack limit K_(MIN) is identified as the colorant value of the minimumblack patch that has an L-value that is less than the L-value extractedfor black from press profile 32.

For example, referring to FIG. 10A, cyan test patch A12 is identified asthe minimum cyan patch that has calorimetric values a=−39.72 andb=−50.34 whose magnitudes are greater than the correspondingcalorimetric values extracted from press profile 32 for maximum cyan(i.e., a=−36 and b=−50 (Table 1)). The cyan value of test patch A12 is47%. Therefore, C_(MIN)=47%. Referring to FIG. 10B, magenta test patchB6 is the minimum magenta patch that has calorimetric values a=75.23 andb=−21.83 whose magnitudes are greater than the magnitudes of thecorresponding colorimetric values extracted from press profile 32 formaximum magenta (i.e., a=75 and b=−7 (Table 1)). In addition, magentatest patch B1 is the minimum magenta patch that has colorimetric valueL=46.58 that is less than the corresponding colorimetric value extractedfrom press profile 32 for maximum magenta (i.e., L=47). The magentavalue of test patch B1 is 75% and B6 is 53%. Therefore, the lower ofthese two values is M_(MIN)=53%. Referring to FIG. 10C, yellow testpatch B15 is the minimum yellow patch that has a calorimetric valueb=95.89 whose magnitude is greater than the magnitude of thecorresponding colorimetric value extracted from press profile 32 formaximum yellow (i.e., b=95 (Table 1)). The yellow value of test patchB15 is 80%. Therefore, Y_(MIN)=80%. Referring to FIG. 10D, black testpatch H3 is the minimum black patch that has a calorimetric valueL=16.80 that is less than the corresponding colorimetric value extractedfrom press profile 32 for maximum black (i.e., L=18 (Table 1)). Theblack value of test patch H3 is 42%. Therefore, K_(MIN)=42%.

Referring now to FIG. 8B, an exemplary method 102 for determining amaximum per-channel colorant limit is described. Steps 110 and 112 havealready been described above in connections with FIG. 8A. At steps 118a-d, the maximum limits C_(MAX), M_(MAX), Y_(MAX) and K_(MAX) aredetermined for the C, M, Y and K colorants, respectively. For the C, Mand Y channels, the maximum limit C_(MAX), M_(MAX) and Y_(MAX), isidentified as the colorant value of the minimum cyan, magenta and yellowpatch, respectively, where: (a) the chroma is maximum, or (b)oversaturation begins. The maximum limit K_(MAX) is identified as thecolorant value of the minimum black patch where: (a) the L-value isminimum, and (b) the difference between the L-value of the patch and theL-value of the 100% patch (L₁₀₀) is sufficiently small. For example, thedifference is sufficiently small if the following inequality issatisfied:(Int[L(i)]−Int[L ₁₀₀])<1  (1)where Int[L(i)] is the integer portion of L(i) and Int[L₁₀₀] is theinteger portion of L₁₀₀.

For C, M and Y colorants, chroma increases with increasing colorantvalue, but may begin to decrease beyond a certain colorant amount. As aresult of rounding errors and device tolerances in measurementequipment, measured chroma values may fluctuate slightly about a maximumvalue. To avoid selecting a false maximum, therefore, the chroma valueof each test patch is compared to the chroma values of severalsuccessive patches. For example, for a series of chroma values, themaximum chroma value may be identified as the chroma value thatsatisfies at least one of the following inequalities:chroma(i)>chroma(i+1)>chroma(i+2)  (2a)orchroma(i)>chroma(i+1)>chroma(i+3)  (2b)In this case, the maximum chroma value must be greater than the firstand second successive chroma values, or the first and third successivechroma values.

To determine if C, M or Y colorants are oversaturated, X, Y and Zcalorimetric values may be used. In particular, because X-values areproportional to red reflection, and because cyan absorbs red, X-valuesideally decrease with increasing amounts of cyan colorant. If cyanbecomes oversaturated, however, X-values may begin to increase withincreasing cyan colorant. As a result, the start of oversaturation ofcyan may be identified by determining the colorant value of the patchwhere X-values begin to increase with increasing cyan colorant.Similarly, Y-values ideally decrease with increasing amounts of magentacolorant, and Z-values ideally decrease with increasing amounts ofyellow colorant. As a result, the start of oversaturation of magenta maybe identified by determining the colorant value of the patch whereY-values begin to increase with increasing magenta colorant, and thestart of oversaturation of yellow may be identified by determining thecolorant value of the patch where Z values begin to increase withincreasing yellow colorant.

As a result of rounding errors and device tolerances in measurementequipment, X, Y and Z measurements may fluctuate slightly about aminimum value. To avoid selecting a false minimum, therefore, theX-value of each test patch is compared to the X-values of severalsuccessive patches. For example, the start of oversaturation of cyan maybe identified as the colorant patch having an X-value X(i) thatsatisfies at least one of the following inequalities:X(i)<X(i+1)<X(i+2)  (3a)orX(i)<X(i+1)<X(i+3)  (3b)In this case, the X-value must be less than the first and secondsuccessive X-values, or the first and third successive X-values.Similarly, the start of oversaturation of magenta may be identified asthe colorant patch having an Y-value Y(i) that satisfies at least one ofthe following inequalities:Y(i)<Y(i+1)<Y(i+2)  (4a)orY(i)<Y(i+1)<Y(i+3)  (4b)and the start of oversaturation of yellow may be identified as thecolorant patch having a Z-value Z(i) that satisfies at least one of thefollowing inequalities:Z(i)<Z(i+1)<Z(i+2)  (5a)orZ(i)<Z(i+1)<Z(i+3)  (5b)

For K, the minimum L-value may be identified as the L value L(i) thatsatisfies the equation:L(i)≦L(i+1)  (6)

Referring again to FIGS. 10A-10D, the above principles may be used todetermine C_(MAX), M_(MAX), Y_(MAX) and K_(MAX). In particular,referring to FIG. 10A, the chroma value of cyan test patch A8 satisfiesequation (2a), and therefore the maximum chroma (68.49) corresponds to acyan colorant value of 65%. In addition, none of the X-values satisfyequation (3), and therefore cyan is not oversaturated. As a result,C_(MAX)=65%. Referring to FIG. 10B, the chroma value of magenta testpatch A22 satisfies equation (2a), and therefore the maximum chroma(82.55) corresponds to a magenta colorant value of 80%. In addition,none of the Y-values satisfy equation (4), and therefore magenta is notoversaturated. As a result, M_(MAX)=80%. Referring to FIG. 10C, thechroma value of all yellow test patches satisfy equation (2a), andtherefore the maximum chroma (102.15) corresponds to a yellow colorantvalue of 100%. In addition, none of the Z-values satisfy equation (5),and therefore yellow is not oversaturated. As a result, Y_(MAX)=100%.Referring to FIG. 10D, the L-value of black test patch H12 satisfiesequations (1) and (6), and therefore the maximum limit for black isK_(MAX)=69%. Thus, following are minimum and maximum limits from theexemplary colorant data of FIGS. 10A-10D:

Colorant Min Limit Max Limit Cyan 47% 65% Magenta 53% 80% Yellow 80%100%  Black 42% 69%

Referring now to FIG. 8C, an exemplary method 104 for determining anoptimal per-channel colorant limit is described. Steps 110 and 112 havealready been described above in connections with FIG. 8A. Beginning atstep 120, the optimal limit C_(OPT), M_(OPT), and Y_(OPT) is determinedfor the C, M and Y colorants, respectively. In particular, colorantvalues are determined for a plurality of “interpolated patches,”consisting of combinations of the cyan, magenta and yellow colorantsused to determine the minimum and maximum colorant values determined insteps 100 and 102. Thus, in the examples illustrated in FIGS. 10A-10C,interpolated patches are created using combinations of the followingcyan, magenta and yellow colorants:

-   -   Cyan: 47%, 53%, 56%, 60% and 65%    -   Magenta: 53%, 56%, 60%, 65%, 70%, 75% and 80%    -   Yellow: 80%, 85%, 90%, 95% and 100%        Colorant values for such interpolated patches are illustrated in        FIGS. 10H-10I.

Referring again to FIG. 8C, at step 122, the chroma value of eachinterpolated patch is calculated based on the specified colorant values.Any conventional technique for determining chroma values based oncolorant values may be used. For example, the well-known classicalNeugebauer equations may be used to calculate the chroma values for eachinterpolated patch. At step 124, chroma values are calculated for themulti-colorant patches printed in step 110, and illustrated in FIGS.10E-10G, using the same technique used in step 122. Thus, for example,if the classical Neugebauer equations were used at step 122, they arealso used at step 124. Next, at step 126, any deviation is determinedbetween the chroma values calculated at step 124, and the chroma valuesof the multi-colorant patches measured at step 112. At step 128, thechroma values of the interpolated patches calculated at step 122 arecorrected using any deviation determined at step 126. For example,well-known vector-corrected Neugebauer equations may be used to correctthe calculated chroma values. FIGS. 10H-10I illustrate vector-correctedchroma values for each of the interpolated patches. Finally, at step130, the interpolated patch having the minimum vector-corrected chromavalue is identified. In the example illustrated in FIGS. 10H-10I, theminimum vector-corrected chroma value is 0.13, corresponding to aninterpolated patch having CMY values (47, 56, 85). Thus, the optimalcyan, magenta and yellow colorant values that provide the bestgray-balance are: C_(OPT)=47%, M_(OPT)=56% and Y_(OPT)=85%.

Referring again to FIG. 8C, at step 132, the optimal limit K_(OPT) isdetermined for the K colorant, which is equal to a weighted average ofthe minimum and maximum limits. An exemplary optimal limit for K may beexpressed as:K _(OPT)=(α×K _(MIN))+(β×K _(MAX))  (7)where α and β are weighting factors. Exemplary values for the weightingfactors are α=0.7 and β=0.3. Thus, from the exemplary minimum andmaximum limits K_(MIN)=42% and K_(MAX)=69%, respectively, K_(OPT)=50%.Persons of ordinary skill in the art will understand that other valuesmay be used for α and β.Linearization Table Calculation

Referring now to FIGS. 3 and 11, exemplary methods and apparatus inaccordance with this invention are described for calculating colorantlinearization tables. In particular, FIG. 11 illustrates an exemplarymethod for calculating a colorant linearization table for a singlecolorant (e.g., Y). Persons of ordinary skill in the art will understandthat the method may be repeated for each colorant used by proofingprinter 46. Beginning at step 150, proofing printer 46 is used to printtest pattern 82 including test patches P on output page 74. An exemplaryoutput page 74 c including exemplary test pattern 82 c is illustrated inFIG. 12. Test pattern 82 c includes two strips of test patches, witheach strip including fifteen test patches P. Persons of ordinary skillin the art will understand that test pattern 82 c may include more orless than two strips, and each strip may include more or less thanfifteen test patches P. Each test patch P is comprised of acorresponding specified percentage of the colorant for which thelinearization table is being created (Y in this example).

FIG. 13 illustrates exemplary colorant values (in percent) for yellowtest patches P (patches are identified in each table by row (A-B) andcolumn number (1-15)). The exemplary values provide test patches Phaving scales of single-colorant values. Persons of ordinary skill inthe art will understand that other specific colorant values also may beused for test patches P. Referring again to FIG. 11, at step 152,colorimetric values are determined for each test patch P printed in step150. For example, measurement device 76 may be used to determine XYZdata for each test patch P on output page 74 c. FIG. 13 illustratesexemplary measured XYZ data for test patches P.

Referring again to FIG. 11, at step 154, tonal response values arecalculated from the measured calorimetric values from step 152. Aspreviously mentioned, X-values are inversely proportional to amounts ofcyan colorant, Y-values are inversely proportional to amounts of magentacolorant, and Z-values are inversely proportional to amounts of yellowcolorant. In addition, Y-values are inversely proportional to amounts ofblack colorant. As a result, the tonal response of a cyan colorant patchP of percent i may be expressed as:

$\begin{matrix}{{{Tonal}\mspace{14mu}{Value}} = \frac{\left( {1 - \frac{X_{i}}{X_{W}}} \right)}{\left( {1 - \frac{X_{100}}{X_{W}}} \right)}} & \left( {8a} \right)\end{matrix}$where X_(i) is the X-value for patch P, X_(W) is the X-value of paperwhite, and X₁₀₀ is the X-value of solid cyan (i.e., C=100%).

Similarly, the tonal response of a magenta colorant patch P of percent imay be expressed as:

$\begin{matrix}{{{Tonal}\mspace{14mu}{Value}} = \frac{\left( {1 - \frac{Y_{i}}{Y_{W}}} \right)}{\left( {1 - \frac{Y_{100}}{Y_{W}}} \right)}} & \left( {8b} \right)\end{matrix}$where Y_(i) is the Y-value for patch P, Y_(W) is the Y-value of paperwhite, and Y₁₀₀ is the Y-value of solid magenta (i.e., M=100%).Likewise, the tonal response of a yellow colorant patch P of percent imay be expressed as:

$\begin{matrix}{{{Tonal}\mspace{14mu}{Value}} = \frac{\left( {1 - \frac{Z_{i}}{Z_{W}}} \right)}{\left( {1 - \frac{Z_{100}}{Z_{W}}} \right)}} & \left( {8c} \right)\end{matrix}$where Z_(i) is the Z-value for patch P, Z_(W) is the Z-value of paperwhite, and Z₁₀₀ is the Z-value of solid yellow (i.e., Y=100%).Similarly, the tonal response of a black colorant patch P of percent imay be expressed as:

$\begin{matrix}{{{Tonal}\mspace{14mu}{Value}} = \frac{\left( {1 - \frac{Y_{i}}{Y_{W}}} \right)}{\left( {1 - \frac{Y_{100}}{Y_{W}}} \right)}} & \left( {8d} \right)\end{matrix}$where Y_(i) is the Y-value for patch P, Y_(W) is the Y-value of paperwhite, and Y₁₀₀ is the Y-value of solid black (i.e., K=100%). FIG. 13illustrates exemplary tonal values calculated based on measured XYZ datafor yellow test patches P. FIG. 14 illustrates a graph of the calculatedtonal values versus input values, with curve 170 representing thecalculated tonal values of FIG. 13.

Referring again to FIGS. 3 and 11, at step 156, XYZ data are retrievedfrom backwards transform 32 b of press profile 32 for each colorant for0%, 100%, and a midtone value (e.g., 40%, 50% or other similar midtonevalue). Table 2 illustrates exemplary XYZ values extracted from pressprofile 32:

TABLE 2 C M Y K X Y Z 100 0 0 0 15.22 22.09 55.16 0 100 0 0 32.36 16.3715.04 0 0 100 0 68.41 73.38 5.98 0 0 0 100 2.22 2.36 1.94 40 0 0 0 52.7158.79 70.22 0 40 0 0 60.90 53.95 51.39 0 0 40 0 76.98 81.94 43.15 0 0 040 39.55 41.03 37.38 0 0 0 0 85.67 88.32 78.70

Next, at step 158, the tonal response of press 34 is calculated usingthe XYZ data retrieved from press profile 32 in step 156, and equations8(a)-8(d). Using the exemplary values in Table 2, the tonal response foryellow is:

TABLE 3 Input Value (%) Tonal Value (%) 0 0 40 49 100 100

At step 160, the tonal response of press 34 is calculated for the entirerange of input values from 0-100% based on the tonal values calculatedin step 158. For example, a spline function may be used to calculatetonal response values for the entire range of input values using thethree data points in Table 3. Curve 172 in FIG. 14 represents the tonalresponse values calculated in step 160. Referring again to FIG. 11, atstep 162, linearization tables are created by mapping press input valuesto equivalent printer input values using the tonal response data. Forexample, using tonal response curves 170 and 172, input values of press34 are mapped to corresponding input values for proofing printer 46 thathave equivalent tonal responses. The process of FIG. 11 may be repeatedfor each colorant used by proofing printer 46.

Distribution of Multi-Hue Colorants

If proofing printer 46 uses light and normal hues of a colorant (e.g.,cyan), the printer may use only light cyan over a first range of inputvalues (e.g., 0-100%), and may use a combination of light and normalcyan over a second range of input values (e.g., 40-100%). Referring nowto FIGS. 3 and 15, exemplary methods and apparatus in accordance withthis invention are described for determining distribution functions ofmulti-hue colorants for such systems. Beginning at step 180, processor78 determines the lower limit L1 of the second range of input values(e.g., 40%), and the amount A1 of light colorant at 100% (e.g., 5% lightcolorant). For example, processor 78 may prompt a user to provide thesetwo values.

Next, at step 182, proofing printer 46 is used to print test pattern 82including test patches P having scales of light colorant only, scales ofnormal colorant only, and combinations of light and normal colorant.Each test patch P is comprised of a corresponding specified percentageof light and normal colorants. Next, at step 184, colorimetric valuesare determined for each test patch P printed in step 182. For example,measurement device 76 may be used to determine XYZ data for each testpatch P. A step 186, tonal response values are calculated from themeasured colorimetric values from step 184. As previously mentioned, forcyan, tonal values are calculated using equation 8(a), for magenta,tonal values are calculated using equation 8(b), for yellow, tonalvalues are calculated using equation 8(c), and for black, tonal valuesare calculated using equation 8(d).

Next, at step 188, the tonal response of press 34 is calculated as insteps 156-160 of FIG. 11, thereby providing target tone values for themulti-hue colorant from 0-100%. At step 190, the light distributionfunction is calculated from 0% to L1 as in step 162 of FIG. 11.Referring again to FIG. 15, at step 192, the light colorant distributionbetween L1 and 100% is determined using any suitable curve fittingtechniques based on the slope of the light colorant distribution curveat L1, the slope of the light colorant distribution curve at 100% (0).

Next, at step 194, a two-dimensional table is created that provides thecalculated tone value for each combination of light and normal colorantcalculated at step 186. Finally, at step 196, the normal colorantdistribution from L1 to 100% is calculated by determining from the twodimensional table the amount of normal that when added to the lightvalue gives the target tone value. Persons of ordinary skill in the artwill understand that the light and normal colorant distribution curvesmay be smoothed using any conventional smoothing algorithm.

The foregoing merely illustrates the principles of this invention, andvarious modifications can be made by persons of ordinary skill in theart without departing from the scope and spirit of this invention.

1. A system for calibrating a digital color imaging device to a printingpress, the printing press comprising a color profile having a pluralityof colorant values and associated colorimetric values, the systemcomprising: the digital color imaging device for printing an imagesource comprising a test pattern, the test pattern comprising aplurality of strips, each strip comprising a plurality of patches, eachpatch comprising a colorant value; an instrument that colorimetricallymeasures the patches in the printed test pattern; and a processor that:(a) receives colorimetric measurements from the instrument andcalculates therefrom a tonal response for the digital color imagingdevice; (b) receives a plurality of colorimetric values from the colorprofile; and (c) calibrates the digital color imaging device to theprinting press by calculating: a total colorant limit by identifying apatch in each strip with a minimum L-value, and from each patch in eachstrip with the minimum L-value, identifying a patch with a maximum totalarea coverage, and setting the total colorant limit to the maximum totalarea coverage, a per-channel colorant limit by determining a minimumlimit for each colorant, a maximum limit for each colorant, and anoptimal per channel colorant limit between the minimum and the maximumlimit for each colorant, and a linearized table for each colorant bydetermining a tonal response for the printing press, and mapping thetonal response of the digital color imaging device to the tonal responsefor the printing press.
 2. The system of claim 1, wherein the maximumlimit for each colorant for the per-channel colorant limit is determinedby: identifying a colorant value of a minimum black patch with a minimumL-value, wherein a difference between an L-value of the minimum blackpatch and the L-value of a 100% black colorant patch is small; andidentifying a colorant value of a printed patch having one of thefollowing: a maximum chroma and a colorant where oversaturation beginsif the colorant is for a colorant other than black.
 3. The system ofclaim 1, wherein the optimal limit for each colorant for the per-channelcolorant limit is determined by: determining a weighted average of theminimum and maximum colorant limits for the black colorant patches; anddetermining a colorant value that provides a gray-balance in the atleast one patch with a combination of multiple-colorant values thatcomprise colorants other than black.
 4. The system of claim 1, whereinthe processor further calibrates the digital color imaging device to theprinting press by calculating a distribution of multi-hue colorants. 5.The system of claim 4, wherein the test pattern includes test patcheshaving scales of light colorant only, scales of normal colorant only,and combinations of light and normal colorant and the distribution ofmulti-hue colorants is calculated by: determining a lower limit (L1) ofa range of input values, determining an amount of light colorant at100%, determining a light colorant distribution function from 0% to L1,and determining a light colorant distribution between L1 and 100%,creating a table that provides a calculated tone value for eachcombination of light and normal colorant, and determining the normalcolorant distribution from L1 to 100%.
 6. The system of claim 1, whereinthe test pattern comprises patches having cyan, magenta, yellow andblack colorants.
 7. The system of claim 1, wherein the digital colorimaging device is adapted to use multi-hue colorants.
 8. The system ofclaim 1, wherein a colorimetrically measuring comprises measuring theprinted test pattern using a colorimeter.
 9. The system of claim 1,wherein colorimetrically measuring comprises measuring the printed testpattern using a spectrophotometer.
 10. The system of claim 1, whereinthe colorimetric measurements comprise CIELAB data values.
 11. Acomputer-implemented method for calibrating a digital color imagingdevice to a printing press, the printing press comprising a colorprofile having a plurality of colorant values and associatedcolorimetric values, comprising the steps of: printing an image sourcecomprising a test pattern, the test pattern comprising a plurality ofstrips, each strip comprising a plurality of patches, each patchcomprising a colorant value; colorimetrically measuring the patches inthe printed test pattern; receiving, with a processor, colorimetricmeasurements; calculating, with the processor, a tonal response for thedigital color imaging device; receiving, with the processor, a pluralityof colorimetric values for the color profile; and calibrating, with theprocessor, the digital color imaging device to the printing press bycalculating: a total colorant limit by identifying a patch in each stripwith a minimum L-value, and from each patch in each strip with theminimum L-value, identifying a patch with a maximum total area coverage,and setting the total colorant limit to the maximum total area coverage,a per-channel colorant limit by determining a minimum limit for eachcolorant, a maximum limit for each colorant, and an optimal per channelcolorant limit between the minimum and the maximum limit for eachcolorant, and a linearized table for each colorant by determining atonal response for the printing press, and mapping the tonal response ofthe digital color imaging device to the tonal response for the printingpress.
 12. The method of claim 11, wherein the maximum limit for eachcolorant for the per-channel colorant limit is determined by:identifying a colorant value of a minimum black patch with a minimumL-value, wherein a difference between an L-value of the minimum blackpatch and the L-value of a 100% black colorant patch is small; andidentifying a colorant value of a printed patch having one of thefollowing: a maximum chroma and a colorant where oversaturation beginsif the colorant is for a colorant other than black.
 13. The method ofclaim 11, wherein the optimal limit for each colorant for theper-channel colorant limit is determined by: determining a weightedaverage of the minimum and maximum colorant limits for the blackcolorant patches; and determining a colorant value that provides agray-balance in the at least one patch with a combination ofmultiple-colorant values that comprise colorants other than black. 14.The method of claim 11, wherein the processor further calibrates thedigital color imaging device to the printing press by calculating adistribution of multi-hue colorants.
 15. The method of claim 14, whereinthe test pattern includes test patches having scales of light colorantonly, scales of normal colorant only, and combinations of light andnormal colorant and the distribution of multi-hue colorants iscalculated by: determining a lower limit (L1) of a range of inputvalues, determining an amount of light colorant at 100%, determining alight colorant distribution function from 0% to L1, and determining alight colorant distribution between L1 and 100%, creating a table thatprovides a calculated tone value for each combination of light andnormal colorant, and determining the normal colorant distribution fromL1 to 100%.
 16. The method of claim 11, wherein the test patterncomprises patches having cyan, magenta, yellow and black colorants. 17.The method of claim 11, wherein the digital color imaging device isadapted to use multi-hue colorants.
 18. The method of claim 11, whereina colorimetrically measuring comprises measuring the printed testpattern using a colorimeter.
 19. The method of claim 11, whereincolorimetrically measuring comprises measuring the printed test patternusing a spectrophotometer.
 20. The method of claim 11, wherein thecolorimetric measurements comprise CIELAB data values.